Bisoxazoline (BOX) Ligand-Metal Complexes: An Emerging Chiral Catalyst

C₂-symmetric chiral bisoxazoline (BOX) ligand-metal complexes have emerged as an effective catalyst for carrying out a wide range of enantioselective reactions. [1] The presence of a C₂-symmetric axis in the bisoxazoline ligands minimizes the number of possible transition states in a particular reaction. [2] Of the various chiral bisoxazoline ligands used, ligands (1-7) with a one carbon bridging between the oxazoline rings are most frequently utilized. Several research groups have utilized these bisoxazoline ligands in combination with a wide range of mild Lewis acids as catalysts for carrying out different enantioselective reactions. [4-13] Recently, chiral BOX-metal(II) complexes covalently anchored to silica and mesoporous MCM-41 have been used as a new heterogenous catalyst for enantioselective Friedel-Crafts hydroxylation. [14] An intriguing feature of the BOX-metal(II) complexes is that the metal(II)-derived chiral Lewis acids shows pronounced counter ion [15a-c] as well as solvent dependence in context of yield, enantioselectivity, rate, and success of reaction. The selectivity observed is due to the fact that the bisoxazoline ligands form six membered metal chelates which are conformationally constrained and the chiral centers in these ligands are located in close proximity to donor nitrogen, thereby imposing a strong directing effect on the catalytic sites. [16]

References

• 1a Ghosh AK. Mathivanen P. Cappiello J. Tetrahedron: Asymmetry  1998,  9:  1 
• 1b Jørgensen KA. Johannsen M. Yao S. Audrain H. Thorhauge J. Acc. Chem. Res.  1999,  32:  605 
• 1c Evans DA. Rovis T. Johnson JS. Pure Appl. Chem.  1999,  71:  1407 
• 1d Johnson JS. Evans DA. Acc. Chem. Res.  2000,  33:  325 
• 1e Jørgensen KA. Angew. Chem. Int. Ed.  2000,  39:  3558 
• 1f Kobayashi S. Manabe K. Acc. Chem. Res.  2002,  35:  209 
• 1g Thorhauge J. Roberson M. Hazell RG. Jørgensen KA. Chem.-Eur. J.  2002,  8:  1888 
• 2 Whitesell JK. Chem. Rev.  1989,  89:  1581 
• 3 Evans DA. MacMillan DWC. Campos KR. J. Am. Chem. Soc.  1997,  119:  10859 
• 4a Evans DA. Johnson JS. Olhava EJ. J. Am. Chem. Soc.  2000,  122:  1635 
• 4b Audrain H. Thorhauge J. Hazell RG. Jørgensen KA. J. Org. Chem.  2000,  65:  4487 
• 4c Zhuang W. Thorhauge J. Jørgensen KA. Chem. Commun.  2000,  459 
• 4d Gothelf AS. Gothelf KV. Hazell RG. Jørgensen KA. Angew. Chem. Int. Ed.  2002,  41:  4410 
• 5 Evans DA. Barnes DM. Johnson JS. Lectka T. von Matt P. Miller SJ. Norcross RD. Shaughnessy EA. Campos KR. J. Am. Chem. Soc.  1999,  121:  7582 
• 6a Evans DA. Burgey CS. Kozlowski MC. Tregay SW. J. Am. Chem. Soc.  1999,  121:  686 
• 6b Reichel F. Fang X. Yao S. Ricci M. Jørgensen KA. Chem. Commun.  1999,  1505 
• 7a Zhuang W. Gathergood N. Hazell RG. Jørgensen KA. J. Org. Chem.  2001,  66:  1009 
• 7b Jensen KB. Thorhauge J. Hazell RG. Jørgensen KA. Angew. Chem. Int. Ed.  2001,  40:  160 
• 7c Zhuang W. Hansen T. Jørgensen KA. Chem. Commun.  2001,  347 
• 8 Audrain H. Jørgensen KA. J. Am. Chem. Soc.  2000,  122:  11543 
• 9 Evans DA. Rovis T. Kozlowski MC. Downey W. Tedrow JS. J. Am. Chem. Soc.  2000,  122:  9134 
• 10 Evans DA. Johnson DS. Org. Lett.  1999,  1:  595 
• 11 Zhang X. Qu Z. Ma Z. Shi W. Jin X. Wang J. J. Org. Chem.  2002,  67:  5621 
• 12 Nishiwaki N. Knudsen KR. Gothelf KV. Jørgensen KA. Angew. Chem. Int. Ed.  2001,  40:  2992 
• 13 Christensen C. Juhl K. Hazell RG. Jørgensen KA. J. Org. Chem.  2002,  67:  4875 
• 14 Corma A. Garcia H. Moussaif A. Sabater MJ. Zniber R. Redouane A. Chem. Commun.  2002,  1058 
• 15a Evans DA. Murry JA. Matt P. Norcross RD. Miller SJ. Angew. Chem. Int. Ed.  1995,  34:  798 
• 15b Johannsen M. Jørgensen KA. Tetrahedron  1996,  52:  7321 
• 15c Johannsen M. Jørgensen KA. J. Chem. Soc., Perkin Trans. 2  1997,  1183 
• 16 For mechanistic aspects see: Morao I. Namara JPM. Hillier IH. J. Am. Chem. Soc.  2003,  125:  628  see also ref 1a,b, 9, 12, 7a
• 17 Evans DA. Peterson GS. Johnson JS. Barnes DM. Campos KR. Woerpel KA. J. Org. Chem.  1998,  63:  4541 
• 18 Juhl K. Gathergood N. Jørgensen KA. Angew. Chem. Int. Ed.  2001,  40:  2995 
• 19 Juhl K. Gathergood N. Jørgensen KA. Chem. Commun.  2000,  2111 
• 20 Schinnerl M. Seitz M. Kaiser A. Reiser O. Org. Lett.  2001,  26:  4259 
• 21 Juhl K. Jørgensen KA. J. Am. Chem. Soc.  2002,  124:  2420